This invention relates in general to mass spectrometers and gas chromatographs (such as gas or liquid types, or direct solid or liquid types) and relates more particularly to an interface that provides a direct path to the ion source and also enables a column of a gas chromatograph to be easily decoupled from the mass spectrometer without cooling and venting or risk of damage to the spectrometer. Modern gas chromatograph/mass spectrometer (GC/MS) instruments utilize gas chromatograph (GC) columns to enhance standard chromatographic results. In such an instrument, a sample is introduced at a pressure on the order of 30 pounds per square inch (psi) into a GC column. The temperature of the GC column is controlled to ramp up to a maximum temperature T sufficiently high to vaporize the entire sample.
In one typical system, the GC column is flexible fused silica having an outside diameter on the order of 0.4 mm and an inside diameter on the order of 0.2-0.3 mm. A vaporized sample is injected into an entrance end of the GC column and the various components of the sample travel along the GC column and out through an exit end of the GC column. The inside of the column has a coating that interacts with the vaporized sample in such a way that the components of the sample travel at various velocities along the column. As a result of the velocity differences, there is a separation in time between the emission of the various components through the exit end of the GC column. The GC column therefore functions to separate the components of the sample. In a GC/MS, the exit end of the capillary column is inserted into the ionization and detection chamber of a mass spectrometer (MS) to identify the various components of the sample.
In the ionization and detection chamber, the vaporized sample components are fragmented (e.g., by bombardment with electrons). The fragments are ionized (e.g., by chemical ionization of electron ionization) and then passed through an electric and/or magnetic field (mass analyzer) that separates the fragments on the basis of their ratio of mass to ionized charge. Each component has a characteristic distribution of quantity of fragments as a function of fragment mass. This distribution serves as a fingerprint that enables identification of each sample component. In order to separate the fragments in the mass analyzer, the pressure in the mass spectrometer is kept low enough (on the order of 10.sup.-6 -10.sup.-5 Torr) that the mean free path length of the ionized fragments is much longer than the typical linear dimension of the mass spectrometer ionization and detection chamber.
There are three common types of interface between the gas chromotagraph and the mass spectrometer. In the first type, known as the capillary direct interface, the capillary column is inserted directly into the mass spectrometer's ionization chamber. This interface has the highest sensitivity because the sample components are injected directly into the ionization and detection chamber. However, it is very time consuming to change the GC column in such an interface. The ionization chamber has electrodes that are at about 250 degrees Centigrade and that will oxydize rapidly if near atmospheric pressure air is allowed into the chamber. Therefore, when the GC column is to be changed, the electrodes in the ionization chamber must be allowed to cool below 100 degrees Centigrade before air is allowed to controllably leak into the ionization chamber--a process that can take on the order of 45 minutes. The vacuum in the ionization chamber must then be reestablished before the mass spectrometer is again used.
The second type of interface, known as the open-split interface, is utilized when it is advantageous to have a larger flow rate in the GC column than is allowed to flow into the mass spectrometer. Typically, the flow rate into the mass spectrometer is limited to less than 1 milliliter/minute. This flow rate may be below the rate that optimizes the GC separation process. To allow this difference in flow rates, this interface utilizes a capillary tube that connects the MS ionization and detection chamber to one end of a cylindrical connection chamber. The exit end of the GC column is inserted into the other end of the connection chamber substantially collinear with the capillary tube. A small gap (on the order of 2.0 mm) is left between the exit end of the GC olumn and the end of the capillary tube to enable the excess flow in the GC column to spill out into the connection chamber. The connection chamber has an exhaust port near one end to allow the excess flow to leave the connection chamber. An inlet port is located near the other end of the connection chamber to allow a purge gas to be supplied to assist removal of the excess gas flow from the connection chamber. For 6 mm packed columns, this approach has the disadvantage of reduced sensitivity because only a portion (on the order of 3%) of the sample enters the MS detector. For capillary columns, this approach has the disadvantage of degradation of the chromatographic resolution because of internal dead volume in the connection chamber.
The third interface type, known as the jet separator interface, is utilized when it is desired to remove lightweight carrier gases from the sample gas before the sample gas enters the mass spectrometer. This action reduces the total gas load on the mass spectrometer vacuum pumps to a manageable flow (throughput Q). The structure of this interface is very similar to that in the open split interface, except that the exit end of the GC column is tapered to produce a jet of the gases emitted from the exit end of the GC column. In many applications, a light carrier gas is utilized in the GC column to carry the sample gas components along the GC column. The molecules of the light carrier gas have a high enough diffusion rate that they diffuse out of the jet, thereby increasing the concentration of sample gas components entering the capillary column. Unfortunately, this interface also has the disadvantage of losing some of the sample gas components in the gap between the GC column and the capillary tube.
In each of these methods, the mass spectrometer can be damaged by the inrush of air if the tubing entering the ionization chamber is broken. It would therefore be advantageous to have a new interface that combines the benefits of the above interfaces and that protects the mass spectrometer from damage.